It is known, for example, to prepare polyisocyanates containing biuret groups from diisocyanates and water (German Pat. No. 1,101,394 and U.S. Pat. No. 3,124,605), hydrogen sulphide (German Pat. No. 1,165,580 and Canadian Pat. No. 757,637), formic acid (German Pat. No. 1,174,760 and U.S. Pat. No. 3,350,438) or tertiary alcohols (German Pat. No. 1,543,178 and U.S. Pat. No. 3,358,010). In these reactions, amino groups are first formed from the isocyanate groups in the diisocyanate starting material and these amino groups react with excess diisocyanate to form the biuret polyisocyanates by way of the corresponding urea diisocyanates.
These known processes have numerous disadvantages. In the heterogeneous reaction of diisocyanates with water, there is a risk of formation of insoluble polyureas which are difficult to separate. Furthermore, the above-mentioned known processes invariably give rise to gaseous by-products, such as carbon dioxide, carbon monoxide, carbon sulphoxide or olefins. Lastly, it is particularly disadvantageous that, in these known processes, some of the isocyanate groups of the diisocyanate used as starting material must be destroyed by amine formation. There has, therefore, been no lack of attempts to prepare polyisocyanates containing biuret groups by direct reaction of polyamines with polyisocyanates without any volatile by-products being split off and without destruction of the isocyanate groups by amine formation. However, owing to the high reactivity of the amino groups with the isocyanate groups, these attempts met with considerable practical difficulties due to the formation of large quantities of insoluble polyureas and cross-linked products. The only processes which met with some success were, therefore, those in which very particular starting materials were used. According to German Auslegeschrift No. 1,215,365 and U.S. Pat. No. 3,441,588, for example, higher molecular weight diamino polyethers must be used as diamine component to prevent the formation of the above-mentioned sparingly soluble by-products. It goes without saying that the necessity first to prepare diamino polyethers by a difficult process cannot be a technically completely satisfactory solution to the problem. The process according to German Offenlegungsschrift No. 1,963,190 and U.S. Pat. No. 3,824,266 is restricted to the use of diprimary aromatic diamines which have reduced reactivity due the steric or electronic effects.
The process according to German Offenlegungsschrift No. 2,261,065 and U.S. Pat. No. 3,903,126 also does not provide a technically feasible method of preparing polyisocyanates containing biuret groups by direct reaction of organic polyisocyanates with simple aliphatic and/or cycloaliphatic polyamines. According to Example 16 of these publications, the preparation of polyisocyanates containing biuret groups from hexamethylene diisocyanate and hexamethylene diamine requires reheating of the reaction mixture at 180.degree. C. for 12 hours to complete the reaction. This long reheating at a high temperature is not only economical but also leads to discoloration of the reaction product, particularly under large scale industrial production conditions, so that the use of these products in light-fast lacquers is strictly limited.
The indication given in general terms in U.S. Pat. No. 3,903,126 that the diamines may be used as mixtures with alcohols, in particular with diols, also does not give any concrete information as to how the difficulties described above could be overcome. In particular, the addition of small quantities of dipropylene glycol as described in Example 19 of the U.S. Pat. No. 3,903,126 affords no advantages over the use of a pure diamine (see Example 2 of the present application).
Although polyisocyanates having a biuret structure, in particular those based on hexamethylene diisocyanate, have attained a position of worldwide technical importance for the manufacture of light-fast and extremely weather-resistant lacquers having maximum gloss retention, polyisocyanates of this type are produced industrially and marketed with a small proportion of monomeric diisocyanate. Extensive toxicological investigations and many years experience in the processing of these products have shown that the upper limit of monomer content (hexamethylene diisocyanate) which may be tolerated in these polyisocyanates is about 0.7%, based on the solids content, because only then may safe working conditions with lacquers produced from them be ensured, provided that, in addition, the usual protective measures for working with lacquers are observed. The above-mentioned limit of about 0.7% has been accepted in the literature, for example, in the memorandum "PUR-Anstrichstoffe" of the deutsche gewerbliche Berufsgenossenschaft and in "Polyurethane Report" of the Paintmakers Association.
Recent extensive investigations have shown that when the above-mentioned polymolecular polyisocyanate mixtures having a biuret structure are stored for prolonged periods, particularly under uncontrolled conditions, for example, during transport in ships in hot climates, this limit of about 0.7% of monomeric hexamethylene diisocyanate is exceeded due to the catalytic action of the walls of the glass or metal containers and due to other as yet unknown catalytic effects and the presence of impurities which cannot be exactly analyzed, so that the monomer content may easily rise above about 1%.
Since it is possible to observe the limit of monomer concentration of about 0.7% under industrial conditions of producing the above-mentioned polyisocyanates and it has been proved for over a decade that such products may be safely processed, it is of the greatest importance for industrial and ecological reasons to increase the stability and reduce the breakdown into monomers and at the same time reduce the viscosity of the known biuret polyisocyanates, which are based on aliphatic or cycloaliphatic diisocyanates and which often have a viscosity of from about 10,000 to 120,000 cP at 20.degree. C. Such reduction in the viscosity makes it possible to prepare one-component and two-component polyurethane lacquers without the use of solvents. Although a process for the preparation of exceptionally low viscosity polyisocyanates having a biuret structure, for example, on the basis of hexamethylene diisocyanate, has been described in U.S. Pat. No. 3,903,127, the polyisocyanates obtainable by this process also have the disadvantage of releasing monomeric hexamethylene diisocyanate in the course of prolonged storage.
It was, therefore, an object of the present invention to provide a process for the preparation of polyisocyanate mixtures containing biuret groups in which the disadvantages of the known processes would be substantially eliminated.
It has surprisingly been found that this problem may be solved in certain mixtures of alcohols, primary amines and, optionally, water, which will be described in more detail below, are used as "biuretizing agents."
By "biuretizing agents" are meant substances which react with organic isocyanates to form biuret groups.